The present invention relates generally to electric resistance heaters and particularly to heater structures for heating alumina bearing material in a chamber adapted to react the alumina of the material with gaseous chlorine.
In making aluminum chloride, for example, a chamber is employed for receiving and holding an alumina bearing material, such as porous alumina particles that have been intimately intermixed with carbon. A stream of gaseous chlorine is directed through the material and chamber to react with the alumina to form a gaseous effluent containing aluminum chloride and carbon oxides. From this effluent the aluminum chloride is recovered. Even though the reaction is exothermic after the reaction has started, an initial input of heat is required to start the reaction, and such initial input of heat is particularly efficient if the source of the heat is disposed directly in the alumina bearing material.
It is expected that a carbonaceous material, such as graphite, because of its generally corrosion and heat resistant characteristics, would be suitable for making a heater or heating elements for use in the environment found in a chlorine-alumina reactor. However, it has been found that graphite is attacked by certain constituents in the reactor in the temperature range of about 200.degree. to 400.degree. C. This is consistent with the findings of Hans Grothe, as discussed on pages 9 to 11 of an article entitled "Electrolysis of Aluminum Chloride" published in Z. Erzbergbau und Metallhuttenwesen 3 (1950), 213-220. Grothe found that "graphite anodes are not destroyed when electrolysis is carried out at rather high temperatures--at least above 450.degree. C.".
An attacking constituent in the above low temperature range in a reactor is believed to be ferric chloride. Ferrous constituents are originally in the alumina bearing material as an impurity, and some ferrous material enters into the reactor via the chlorine directed to the reactor through iron and steel pipes or tubes. In the reacting process ferric chloride is formed and attacks the graphite of the heater by apparently entering the molecular structure of the graphite which weakens the same.
Another problem that has been encountered in developing heaters for reactor chambers has been the substantial expansion of heater components within the reactor when the reactor reaches its operating temperature. In experimenting with elongated graphite cross members (as heaters) attached to two widely spaced apart graphite posts by graphite pins or bolts, the cross members expanded longitudinally to such an extent that the posts, cross members and pins cracked and broke.